A state-of-the-art conversational codec can represent with a very good quality a clean speech signal with a bit rate of around 8 kbps and approach transparency at a bit rate of 16 kbps. To sustain this high speech quality even at low bit rate a multi modal coding scheme may be used. Usually the input sound signal is split among different categories reflecting its characteristics. For example, the different categories may include voiced, unvoiced and onset. The codec uses different coding modes optimized for all these categories.
However, some deployed speech codecs do not use this multi modal approach resulting in a suboptimal quality especially at low bit rates for a sound signal different from clean speech. When a codec is deployed, it is hard to modify the encoder due to the fact that the bitstream is standardized and any modification to the bitstream would break the interoperability of the codec. However modifications to the decoder can be implemented to improve the quality perceived on the receiver side.